Method and electrolyzer for softening water

ABSTRACT

The invention relates to the field of water purification.  A method for  sening water by way of its electrochemical treatment involves causing water to pass in the form of two streams through anode and cathode chambers of a membrane electrolyzer. According to the invention, the water stream being fed to the anode chamber is preliminarily softened before supplying it to the anode chamber, and this stream is in the form of catholyte after the separation of precipitating hardness salts therefrom, anolyte being used as the end product. An electrolyzer for softening water has a casing divided by membranes into anode and cathode chambers, respectively. According to the invention, anodes are spaced from the membrane, and perforation holes of a cathode are in the form of slits extending about perpendicularly with respect to the water flow direction, and projections are provided on the surface of the cathode facing towards the interior of the cathode chamber inbetween the slits, the leading faces of the projections in the flow direction being inclined at an agle of 20° to 40° with respect to the surface of the membranes. 
     The invention may be used in any fields where water softening is required.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to water processing, and, in particular, it isconcerned with a method for softening water by way of an electrochemicaltreatment, and an electrolyzer for carrying out the method.

2. Description of the Related Art

Known in the art is a method for softening water by adding reagents,e.g. lime in the form of lime milk to water being softened. Hardnesssalts turn to insoluble form and can be separated from water beingsoftened in the form of a precipitate by setting or filtering (cf.Construction Norms and Rules. Water Supply. Outside Networks andInstallation (in Russian). SNiP 2.04.84. 1985, Moscow, pp. 108-109). Theknown method does not allow a desired degree of softening of water to beachieved. The residual hardness can only be lowered to 0.5-1 mg-eq./lusing auxiliary methods such as adding soda and heating water to 35°-40°C. In addition, the known method involves the use of reagent addingequipment, mixers, suspended bed clarifiers and apparatuses forstabilizing treatment of water and coagulants. In other words, the knownmethod for softening water using chemicals calls for sophisticatedequipment including a large number of individual components.

Also known in the art is a method for softening water by way ofelectrochemical treatment, comprising causing water to pass in the formof two streams through a membrane electrolyzer, one stream being causedto pass through an anode chamber to produce anolyte, the other streambeing caused to pass through a cathode chamber to produce catholyte fromwhich precipitating hardness salts are separated, combining catholyteand anolyte together and using the joint flow as the end product (cf.Jr. "Vodosnabzhenie i sanitarnaya tekhnika", 1982, No. 4, pp. 7-8). Theprior art method can only lower hardness of water to 0.8-1.2 mg-eq./l,i.e. the method is essentially a method of prepurification of waterprior to, e.g. a sodium-cationite method of water softening, and waterbeing softened is supplied, after its treatment in the membraneelectrolyzer, to an ion-exchange apparatus in which it is treated usingthe sodium-cationite method.

This prior art method calls for the use of chemically pure reagent NaCl,HCl so that the method is rather expensive. In addition, a high waterconsumption for washing is required. This method is generally used forsoftening water with a low hardness.

Known in the art is an electrolyzer for softening water, comprising acasing divided by a membrane into flow-through chambers: an anodechamber having an anode accommodated therein and a cathode chamberhaving a cathode accommodated therein, the anode and cathode being inthe form of plates, pressed against the membrane on either side, andseparated therefrom with respective electrically insulating layers, theanode and cathode and the electrically insulating layers havingperforations (SU, A, 882944).

The prior art electrolyzer provides a poor quality of purification inthe anode chamber as the anode prevents cations from passing from theanode chamber to the cathode chamber, the positively charged ions beingrepelled from the anode, i.e. such electrolyzers are substantially pHelectrocoagulators. In additon, manufacture and assembly of theelectrolyzer call for a high accuracy of coaxial perforation holes sinceif they are misaligned the holes are partly covered so as to reduce theworking surface area of the membrane.

Most intensive electrolysis occurs inside the perforation holes in viewof minimum electrical resistance which is especially pronounced with alarge thickness of electrodes, e.g. when graphite electrodes are used.However, this results in clogging of the membrane in the zones of theholes with electrolysis products so as to lower reliability of theelectrolyzer in operation.

SUMMARY OF THE INVENTION

The invention is based on the problem of providing a method forsoftening water by way of an electrochemical treatment having such asequence of treatment steps as to ensure the production of water with ahardness of 0.2 to 0.3 mg-eq./l in a single softening stage withoutusing auxiliary means and also of providing an electrolyzer for carryingout such a method in which a cathode is so constructed as tosubstantially decrease clogging of perforation holes during theelectrochemical water softening thus enhancing quality of watersoftening.

This problem is solved by a method for softening water by way of itselectrochemical treatment, comprising the steps of causing water to passin the form of two streams through a membrane electrolyzer, one streambeing caused to pass through an anode chamber to produce anolyte, theother stream being caused to pass through a cathode chamber to produce acatholyte from which precipitating hardness salts are separated,wherein, according to the invention, the water stream to be fed to theanode chamber is preliminarily softened before feeding to the anodechamber, and for that purpose this stream is in the form of at least apart of the stream of catholyte after the separation of precipitatinghardness salts therefrom, the anolyte being used as the end product.

The problem is also solved by an electrolyzer for softening water,comprising a casing divided by a membrane into flow-through chambers; ananode chamber accommodating an anode and a cathode chamber accommodatinga cathode which is pressed against the membrane, an electricallyinsulating layer being provided between the cathode and membrane andperforation holes being made in the cathode and electrically insulatinglayer. According to the invention, the anode is positioned in a spacedrelation to the membrane, the perforation holes of the cathode andelectrically insulating layer being in the form of slots extending aboutperpendicularly with respect to the direction of water flow, elongatedprojections being provided on the cathode surface facing towards theinterior of the cathode chamber to extend in between the slits andtherealong, the leading faces of the projections in the direction offlow being inclined at 20° to 40° with respect to the surface of themembrane.

To enlarge the working surface of the membrane, through holes havingtheir axes perpendicular with respect to the surface of the membrane arepreferably provided in the body of the projections.

The method for softening water by way of its electrochemical treatment,according to the invention, allows water with a hardness of 0.2 to 0.3mg-eq./l to be produced in a single treatment stage without usingauxiliary treatment methods such as sodium-cationization; it is simpleand features high reliability.

The electrolyzer for water softening, according to the invention, whichis used to carry out the method, according to the invention, is simplein structure, features a high throughput capacity and reliability inoperation. Positioning the anode in a spaced relation to the membraneallows cations to flow unobstructedly through the membrane into thecathode chamber in which the water stream is preliminarily softened whenit passes through the cathode chamber. The provision of the perforationholes in the form of slits and projections in between the slits havingincluded streamlined surfaces in the direction of flow contributes tothe intensive washing of products of electrolysis off the surfaces ofthe membrane owing to enhanced hydrodynamic conditions of flow in thezone of the perforation holes. The provision of through holes in thebody of the projections ensures an increase in the working surface ofthe membrane, hence, enhances throughput capacity of the electrolyzer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail with reference to theaccompanying drawings, in which:

FIG. 1 is a diagrammatic view of an electrolyzer for softening water,according to the invention, in longitudinal section;

FIG. 2 is a sectional view taken along line II--II in FIG. 1;

FIG. 3 is a sectional view taken along line III--III in FIG. 1;

FIG. 4 is a diagrammatic view of a plant for carrying out a method forsoftening water, according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A method for softening water consists in the following. Water to besoftened is caused to pass in the form of two streams through a membraneelectrolyzer. One stream is caused to pass through an anode chamber toproduce anolyte and the other stream is caused to pass through a cathodechamber to produce catholyte from which precipitating hardness salts areseparated. The stream of water fed to the anode chamber is preliminarilysoftened, and for that purpose this stream is at least partially ofcatholyte after the separation of precipitating hardness saltstherefrom, the anolyte being used as the end product.

The method for softening water, according to the invention, is carriedout using an electrolyzer for softening water shown in FIGS. 1, 2, 3.

An electrolyzer for softening water comprises an insulating rectangularcasing 1 divided by means of two parallel membranes 2 into threeflow-through chambers: a pair of lateral anode chambers 3 and a centralcathode chamber 4 located therebetween. Each anode chamber 3 has ananode 5 in the form of a plate of an electrically conducting materialwhich is positioned at a distance d from the membrane 2. A cathode 6 isbox-shaped to the configuration of the cathode chamber 4 and has itsopposed walls pressed against the membrane 2, an electrically insulatinglayer 7 being provided between each of these walls and the respectivemembrane 2. Perforation holes 8 in the form of slits (which are referredto below as slits 8) are made in the electrically insulating layers 7and in the walls of the cathode 6 adjoining thereto, the slits extendingperpendicularly with respect to the flow of water through the cathodechamber 4. Elongated projections 9 are provided in between the slits 8on the surface of the cathode 6 facing towards the interior of thecathode chamber 4 to extend along the slits 8, the leading faces of theprojections in the direction of flow being inclined at an angleα=20°-40° with respect to the surface of the membrane 2. Through holes10 having their axes perpendicular with respect to the surface of themembranes 2 are made in the body of the projections 9. The cathode 6also has ridges 11.

The electrolyzer shown in FIGS. 1, 2, 3 functions in the followingmanner. Water to be softened is caused to pass through the electrolyzerin the form of three streams: one, central stream is caused to passthrough the cathode chamber 4, and the two lateral streams are caused topass through respective anode chambers 3.

Direct current is supplied to the electrodes--the anodes 5 and cathode 6(the power supply is not shown). Calcium and magnesium ions that makewater hard are caused to move under the action of direct current fromthe anode chambers 3 into the cathode chamber 4 through the membranes 2which are made, e.g. of a belting-type cloth and through perforationholes or slits 8 in the cathode 6 and insulating layers 7. Anolyte isthus softened. Owing to water electrolysis in the cathode chamber 4,catholyte is alkalized to a pH from 10.5 to 11.5. Calcium and magnesiumions turn to insoluble hydroxides and carbonates at the abovementionedpH values. Water electrolysis occurs only on the inner surface of thecathode 6 as the outer surface thereof, which is pressed against themembranes 2, is protected by means of the electrically insulating layers7. As a result, the membranes 2 are not clogged with products ofelectrolysis on the surfaces of the cathode 6 facing towards themembranes 2.

The provision of the projections 9 on the inner surface of the cathode 6in between the slits 8 with the leading faces of the projections in thedirection of flow inclined at an angle α=20°-40° with respect to thesurface of the membrane 2 makes it possible to ensure hydrodynamics ofthe flow such that no dead zones are formed in the slits 8 of thecathode 6, and electrolysis products in the form of precipitatinghardness salts and gas bubbles are readily removed with the flow fromthe surfaces of the membranes 2. The angle α of inclination of the facesof the projections 9 with respect to the surface of the membranes 2 waschosen by way of experiments. If the angle is greater than α=40°, theremoval of electrolysis products is inadequate, and with an angle αsmaller than 20° a substantial overlap (decrease) of the useful area ofthe membranes 2 occurs so that the projections become ineffectual.

The provision of the through holes 10 in the body of the projections 9,the axes of the holes being perpendicular with respect to the surface ofthe membranes 2, allows the working area of the membranes 2 to beenlarged so as to enhance throughput capacity of the electrolyzer.

The method for softening water, according to the invention, was carriedout using a plant shown in FIG. 4.

The plant has a water heater 12 having an inlet connected to a line forsupplying water to be softened and an outlet communicating, via a pairof pipelines having valves 13, 14, respectively, with an electrolyzer 15for softening water which has a cathode chamber 16 and a pair of anodechambers 17. Inlets of the cathode chamber 16 and anode chamber 17communicate with the outlet of the heater 12 through the valves 13, 14,respectively, the outlet of the cathode chamber 16 communicates, via apipeline 18, with an inlet of a filter 19 which has its outletconnected, via a valve 20, to the inlets of the anode chambers 17.Outlets of the anode chambers communicate with a pipeline 21 for theremoval of the end product.

Water from the heater 12 is supplied in the form of two streams throughthe open valves 13 and 14 to the electrolyzer 15 constructed inaccordance with the invention, into the cathode chamber 16 and two anodechambers 17. The valve 20 is shut. Water is fed from the cathode chamber16, via the pipeline 18, to the filter 19. The valve 14 is then shut,the valve 20 is opened, and the softened catholyte, after the separationof precipitating hardness salts in the filter 19, is fed from the outletof the filter 19, via the valve 20, to the anode chambers 17 for finalsoftening, while starting water (hard water) continues to be admitted tothe cathode chamber 16 from the heater 12 through the open valve 13.Hardness of catholyte, after the separation of precipitate in the filter19, is from 2.6 to 3.9 mg-eq./l. Anolyte fed to the pipeline 21 in theform of the end product has a hardness of maximum 0.2 to 0.3 mg-eq./l.

Better understanding of the invention may be had from the followingspecific embodiments of the method for softening water, according to theinvention.

EXAMPLE 1

Water with a hardness of 6.6 mg-eq./l was supplied in the form of threestreams to the electrolyzer 15: two streams to the anode chambers 17 toobtain anolyte at the outlet and the third stream to the cathode chamber16 to obtain catholyte from which precipitating hardness salts wereseparated by filtering. After the separation of hardness salts, pH ofthe catholyte was 11.3, and hardness of the catholyte was 3.9 mg-eq./l.Water supply to the anode chambers 17 was then interrupted, and startingwater with the hardness of 6.6 mg-eq./l continued to be admitted to thecathode chamber 16. After purification of the catholyte occurred in theanode chambers 17, and hardness of the anolyte (end product) at theoutlets of the anode chambers 17 was 0.3 mg-eq./l at pH =6-7.

EXAMPLE 2

Starting water was softened as described in Example 1, with the onlydifference that starting water was heated in the heater 12° to 40° C.before supplying it to the cathode chamber 16. Hardness of the endproduct (anolyte) was lowered to 0.2 mg-eq./l, and power consumption forwater softening decreased by 20-30%.

EXAMPLE 3 (comparison with the prior art)

Conditions for the electrochemical softening of water were the same asin Example 1, but catholyte was not supplied to the anode chambers 17for afterpurification after the separation of hardness salts therefrom;it was rather mixed with anolyte to be used as the end product.

Hardness of catholyte was 0.55 mg-eq./l; hardness of anolyte was 2.1mg-eq./l; hardness of the combined flow (end product) was ##EQU1##

Therefore, the use of the flow of water preliminarily softened (in theform of catholyte) supplied to the anode chambers makes it possible toproduce water with a hardness of 0.2 to 0.3 mg-eq./l in a singletreatment stage. The lower limit can be achieved by heating startingwater to 40° C. without additional power consumption as industrialeffluents are generally heated to 40°-60° C. when they are supplied forsoftening.

The invention may be used in power engineering, chemistry,pharmaceutical industry and in other fields where water softening isrequired.

We claim:
 1. A method for softening water by electrochemical treatment,comprising the steps of: causing water to pass in the form of twostreams through a membrane electrolyzer; causing one stream to passthrough an anode chamber to produce anolyte; causing the other stream topass through a cathode chamber to produce catholyte from whichprecipitating hardness salts are separated; preliminarily softening thewater stream to be fed to the anode chamber before supplying it to theanode chamber, by using at least a part of the catholyte stream afterthe separation of precipitating hardness salts therefrom, the anolytebeing used as the end product.